1. Field of the Invention
This invention relates to integrated circuits, and more particularly, to amplifiers implemented as integrated circuits.
2. Description of the Related Art
The basic function of an amplifier is to produce and output signal whose power is a multiple of the power of an input signal. In many applications it is desirable that the output waveform faithfully reproduce the shape of the input signal while magnifying its voltage and/or current in a linear fashion. Traditionally, amplifiers designed for these types of applications have been configured for class A operation.
In an amplifier designed for class A operation, both output devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers in reality are not complementary designs. They are single-ended designs with only one type polarity output devices. They may have “bottom side” transistors but these are operated as fixed current sources, not amplifying devices.
Since a class A amplifier operates from only one power supply, the voltage level of the supply must be somewhat greater than the level of the peak output specified. Therefore, during times in which the input signal is very small, the difference between the amplitude of the output signal and the voltage of the power supply will be large. The amount of non-usable power to be dissipated in the output devices is the aforementioned voltage difference multiplied by the output current. Even in those instances where the output is at its maximum level, there will still be a non-negligible voltage drop across the output devices and corresponding level of non-usable power dissipated in the devices.
Consequently class A is the most inefficient of all power amplifier designs, averaging only around 20% (meaning it consumes about 5 times as much power from the source as it delivers to the load!) Thus class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power. The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion.
In order to increase the efficiency of an amplifier while maintaining a high degree of linearity, a class G design may be employed. Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that under most circumstances, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight.
Typically, class G amplifier implementations employ current blocking diodes to prevent current from being driven into a lower voltage supply when the amplifier output exceeds the lower supply voltage. This effectively protects the lower voltage power supplies, but also limits the efficiency of their contribution to the amplifier output. The power dissipated in the diode will be the voltage drop across the diode times the output current. This power loss will occur any time a lower voltage supply is contributing to the output of the amplifier. In addition, each output stage normally includes a power device to control the flow of current to the load. This device dissipates power equal to the difference between the supply voltage and the amplifier output multiplied by the load current. Again, this power will be wasted any time the supply is contributing to the amplifier output.
When a power supply is contributing maximum current to the amplifier output, the output device will typically be saturated and drop a few tenths of a volt. When added to the diode drop for a lower voltage supply, the total difference between the supply voltage and the amplifier output may be around one volt. While such a voltage drop and corresponding inefficiency may be acceptable in a relatively high voltage amplifier design where the output is several tens of volts, integrated circuit amplifiers for low-power applications are typically designed to operate with minimum supply voltages below two volts and such a drop in output stage voltage would limit the amplifier's maximum efficiency to less than fifty percent. Therefore, a more efficient design for a class G amplifier may be desirable.